Linked from: Configuring Partitions and RAID
For software RAID 1, you need two hard disks. Since the RAID partitions on both the hard disks must be of equal size, it is a good idea to use two hard disks with (roughly) the same storage capacity.
Choose 'Create custom layout' at the menu in Disk Druid. Once you are in the Partitioning tool you will be able to navigate and change the drive partitions.
In our example, we are using two SATA disks on two different disk controllers. These hard disks are detected in Linux as:
Please refer to Deleting Partitions if you need assistance in removing existing partitions.
After all the partitions are deleted, we can start our RAID configuration. First, we are going to start with the swap memory partitions. Putting swap memory on a software RAID partition is not recommended. For this reason, simply create swap partitions on both hard disks.
Repeat the same process, but this time mark sdc as an allowable drive and take the mark off of sda.
The boot partition (/boot) is where we are going to start with our RAID solution.
Repeat the same process, but this time mark sdc as an allowable drive and take the mark off of all the other drives. Now that we have two identical 100 MB partitions on both disks, we can create the software RAID disk:
This example creates the /boot partition. Go through the same process for the root partition (/) and optionally any other partition that you want to create (/home, /var, etc.).
Once completed, click OK and continue through the boot loader installation process. Please refer to Boot Loader document for if you require help with this step.